1. Field of the Invention
This invention relates to a method and device for demounting in a radiation detector a photomultiplier tube which is bonded with a scintillation crystal assembly by means of an elastic light transparent adhesive. A preferred field of application of the invention is a scintillation gamma camera as radiation detector, which comprises a plurality of photomultiplier tubes connected with an scintillation crystal assembly.
2. Description of the Prior Art
Conventional radiation detectors which comprise merely one photomultiplier tube normally make use of epoxy resin as adhesive for coupling the sole photomultiplier tube to the scintillation crystal assembly. Epoxy resin, however provides a rigid bond (see for example the brochure Harshaw Scintillation Phosphors D-4450 of the Harshaw Chemical Company, Crystal & Electronic Products Department, Solon, Ohio, page 43, left column, lines 3 and 4). Due to this, in the case the sole photomultiplier tube becomes defective, a decoupling and replacement of the latter one is not possible. Thus usually the complete radiation detector comprising the defective photomultiplier tube and the scintillation crystal assembly has to be thrown away.
This procedure, however is not practicable for those radiation detectors which comprise a plurality of photomultiplier tubes, such as for example for a scintillation gamma camera. It would become too expensive to throw away a complete camera head merely for that reason that one sole photomultiplier tube had become defective. Due to this in conventional radiation detectors such as scintillation gamma cameras which comprise an assembly of photomultiplier tubes, use is made of an silicone compound with grease-like properties (e.g. silicone oil as for example described in section 2.1 of the brochure "Oken Synthetic Optical Crystals and Scintillation Phosphors" of Ohyo Koken Kogyo Co., Tokyo, Japan) as optical coupling medium for coupling the photomultiplier tubes with the scintillation crystal assembly. This conventional optical coupling compound, however remains semi-fluid and requires mechanical constraint and pressure loading to reduce the rate of spontaneous decoupling, as described for example in the U.S. Pat. Nos. 3,723,735 and 4,280,051, where the photomultiplier tubes have to be spring biased. Even with extensive care, spontaneous decoupling occurs occassionally after installation of a detector unit.
According to the U.S. Pat. No. 4,029,964 other light transparent adhesives, such as for example silicone rubber, have been practiced within an scintillation crystal assembly to couple a light conduction element such as a light pipe with a bilateral glass cover disc for the scintillation crystal. The light transparent silicone rubber is applied in liquid or gel form and is therafter cured. However, to secure the photomultiplier tubes in optical communication with the light conducting element of the scintillation crystal assembly again a conventional grease-like optical coupling compound is used to couple the photomultiplier tubes with the scintillation crystal assembly.
A radiation detector, which comprises two scintillation crystals which are bonded by means of a resilient silicone rubber compound to a photomultiplier tube is described in the U.S. Pat. No. 4,323,778. However, as shown in FIG. 3 of this U.S. patent both the scintillation crystals and the photomultiplier tube are completely embedded in the silicone rubber compound. A decoupling of the photomultiplier tube is only possible by destroying of the complete assembly of photomultiplier tube and scintillation crystal. As namely described on page 1, lines 29 to 30 of the German laid-open specification No. 2,519,034 or as mentioned in col. 1, lines 42 and 47, 48 of the U.S. Pat. No. 2,996,419 silicone rubber despite its casting elasticity has a very high adhesive strength. Thus a decoupling without destruction of photomultiplier tube and/or scintillation crystal assembly seemed to be unimaginable.